1. Field of the Invention
The present invention relates to a multi-slice or cone-beam X-ray computed tomography apparatus.
2. Description of the Related Art
A cone-beam CT apparatus has a larger number of detector arrays and a wider X-ray divergence angle in the slice direction than a multi-slice CT apparatus. Typically, the number of detector arrays mounted in a multi-slice CT apparatus is, for example, 4, 8, or 16, whereas that in a cone-beam CT apparatus reaches as many as 256 or 512.
Such a cone-beam CT apparatus can perform “helical scan” like a single-slice CT apparatus or multi-slice CT apparatus. Helical scan is a technique of obtaining data while an X-ray tube and X-ray detector relatively move in the slice direction (substantially parallel to the body axis direction of an object to be examined) while rotating around the object. In helical scan, the X-ray tube moves along a helical path around the object. Helical scan allows acquisition of data in a wide range within a short period of time.
As indicated by the hatching in FIG. 1, in helical scan, an examiner sets an FOV (Field Of View). Image data is reconstructed within the field of view. A field of view is synonymous with a reconstruction area.
In helical scan, an X-ray tube 111 generates X-rays in a section where it moves, together with a detector 112, by a distance X relative to an object to be examined. This distance X is set to be equivalent to a length H of a reconstruction area FOV. This results in unnecessary areas (shaded portions) irradiated with X-rays in spite of the fact that no image data is reconstructed outside the reconstruction area FOV. As shown in FIG. 2, these unnecessary areas are produced regardless of radiuss S and SS.
The following problem arises in a cone-beam CT apparatus. In the case of a single-slice CT apparatus, as shown in FIG. 3, the operator sets the width (radius or radius; radius in this case) of the circular reconstruction area FOV, which actually has a thin cylindrical shape having a thickness, and a slice thickness, in addition to a tube voltage, tube current, scan time, and the like. The opening degree of a X-ray stop (collimator) 102 for limiting the divergence angle (called the cone angle) of X-rays from an X-ray tube 101 in the slice direction is adjusted such that the thickness of an X-ray beam coincides with the set slice thickness on a rotational axis (Z-axis). At this time, the peripheral portions of the reconstruction area FOV which are indicated by the hatching in FIG. 3 are irradiated with no X-rays. That is, the data of these portions are omitted from the corresponding areas (hatched portions) when each view is taken into account. Although the data is acquired in a view at an opposite direction, this data omission affects the image quality of the peripheral portions of an MPR image. In practice, however, in single-slice CT, the volume ratio of the data omission portion to the reconstruction area FOV is very much limited, and hence no significant problem arises.
This problem, however, becomes evident in cone-beam CT. FIG. 4 schematically shows the geometrical relationship between an X-ray irradiation range and a reconstruction area. Of the three slices, a central slice S2 has almost no data omission portion. In two end slices S1 and S3, data omission occurs in most of the peripheral portions indicated by the hatching. As shown in FIG. 5, therefore, in order to prevent image deterioration in the end slices S1 and S3, studies have been made to set the opening degree of the X-ray stop 102 to a slightly large value in accordance with the length of a virtual reconstruction area longer than the actual reconstruction area by a fixed value ΔW.
Image deterioration can be suppressed to some extent by this opening degree setting method. As shown in FIG. 6, however, when the reconstruction area FOV is set to a small radius, areas outside the reconstruction area FOV are excessively irradiated with X-rays, resulting in an increase in X-ray dose.